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How the 'terror bird' tore its prey

Millions of years ago, the most fearsome predator in South America wasn't Tyrannosaurus rex or a raging mammoth - but a flightless bird with an enormous beak. The creatures known as "terror birds" held sway starting about 60 million years ago, and dominated the continent until only about 2 million years ago. But it wasn't clear exactly how the terror birds killed their prey. Until now.

Paleontologists can now say with confidence that the terror birds, known formally as phorusrhacids, wielded their beaks like hatchets. They repeatedly hacked at their foes, rather than chomping at them and shaking them from side to side as T. rex might have done.

These birds weren't built to wade into the fray like a feathered version of former boxing champ Joe Frazier. Instead, they took after Muhammad Ali, who famously defeated Frazier by floating like a butterfly and stinging like a ... well, like a terror bird.

Paleontologists can say all this thanks to a detailed analysis of three-dimensional X-ray scans, documenting the structure of a terror bird's 6 million-year-old skull. The results were reported today in the open-access journal PLoS ONE.

"One of the things that was surprising about this study is that we were actually able to find out quite a bit," one of the study's co-authors, Lawrence Witmer of the Ohio University College of Osteopathic Medicine, told me in advance of today's publication.

Witmer has been studying the fossilized skulls of terror birds and other ancient creatures for some time now, trying to figure out how their brains worked. When he heard that other researchers were also looking into how terror birds did their thing, he linked up with them and providing the CT scans of a skull from a phorusrhacid species known as Andalgalornis. The scans allowed researchers to create a computer model suggesting what the skull was capable of doing ... as well as what it couldn't do.

The skulls of modern-day birds have lots of light, mobile joints. Not so for the terror bird: "They evolutionarily transformed many of these joints into rigid, dense beams and struts," Witmer explained.

The structure was optimized for a downward hacking motion of the beak. The bird could also conceivably bite down and crush its prey. But there was no way it could hang on and shake its victim from side to side. "It actually looked like that would cause catastrophic failure [of the skull], which is about as dramatic as it sounds," Witmer said.

"A lot of animals just wade into battle, wrestle and battle to subdue their prey," he said. "That's pretty tough. What we found is that weakness from side to side really prevented these animals from doing that going into battle. Instead, it would attack and retreat, attack and retreat, making these almost surgical precision strikes."

Once the terror bird felled its victim, it could tear the critter (most likely some sort of mammal) into bite-size pieces, or just swallow it whole.

Other researchers behind the study - including the lead author, Federico Degrange of the Museo de la Plata/CONICET in Argentina - worked with zookeepers to study the bite strength of an eagle as well as a seriema, a bird thought to be the closest living relative of the terror bird. All the findings were consistent with the Muhammad Ali strategy.

Degrange said the study provides the best explanation yet for why the terror bird was so terrible. "We need to figure out the ecological role that these amazing birds played if we really want to understand how the unusual ecosystems of South America evolved over the past 60 million years," he said in a news release about the research.

Ohio University

The fossil skull of the terror bird Andalgalornis is significantly larger than the skull of a modern-day golden eagle and a human skull. Andalgalornis lived 6 million years ago.

Terror birds had few equals in their ecosystem. Witmer said its closest rival may well have been a saber-toothed marsupial known as Thylacosmilus. The bird that was analyzed for the PLoS ONE study stood about 4.5 feet (1.4 meters tall), but other terror birds are thought to have stood as tall as 7 feet (2.1 meters), dwarfing modern-day eagles. Most of them ranged over South America when it was an island continent, before its tectonic linkup with North America. At least one species, Titanis walleri, invaded North America millions of years ago in the Great American Interchange.

The fall of the terror birds appears to have been as dramatic as its reign, but it's not yet clear exactly what caused them to go extinct. Paleontologists are pretty sure about one thing: Humans didn't kill them off. The birds may have been done in by big cats coming down into South America. Climate change may have played a role as well. "The extinction of successful animals is very rarely due to one factor, but a combination of events," zoologist Ross Piper observed in a posting on Scrubmuncher's Blog.

Witmer said the study published today is a great example showing how paleontologists are "teasing everything we can out of these dusty bones."

"Making these comparisons really starts to give us a broader view of what predators were like," he told me. "We're talking about a different world, a whole different dynamic. It's really a different window on the past, and terror birds wind up providing a pretty exciting view through that window."